1. Field of the Invention
The present invention generally relates to a method and apparatus for performing simultaneous multi-point measurements of multiple velocity components in an unseeded flow.
2. Description of Related Art
Gas flow velocity is an important parameter to measure in scientific, engineering and industrial applications. The velocity field dominates effects such as heat transfer, aerodynamics, and mass transport. Most known methods for measuring the velocity do not work in all environments and, in some cases, require undesirable modifications to the flow. For example, many known techniques require that particles or gasses be seeded into the flow.
Velocity measurement has been performed extensively using particle-based measurement techniques as mentioned above. First, particles are seeded into the flow. Then, Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) is used. However, seeding the flow with particles is undesirable since it is time consuming and increases cost. Furthermore, it is not possible to seed some flows with particles. Additionally, such particles do not always follow the gas flow, especially in supersonic and hypersonic flows and with vortices at any speed.
Molecular-based velocimetry systems use molecules rather than particles. Such molecular-based velocimetry systems include planar laser-induced fluorescence (LIF or PLIF), flow tagging velocimetry (RELIEF), CARS (Coherent anti-Stokes Raman Scattering) velocimetry, laser-induced thermal acoustics (LILA), etc. All of these molecular based known methods have disadvantages or limitations, namely, they: (i) are limited to a small class of flows, (ii) do not work well at high temperatures, (iii) cannot be used to measure three velocity components simultaneously using a single probing laser beam, (iv) sometimes requires specific molecules to be seeded into the flow, and (v) cannot be used easily to measure at multiple points.
Another known technique is iodine-cell-filtered Rayleigh scattering. This technique was developed for unseeded velocimetry in airflows and is used for two-dimensional imaging. This technique uses two cameras to view the flow. One of the cameras looks through an iodine filter. However, this technique has limitations in that the spectroscopy of the iodine is fixed and the technique does not work well at high temperatures (e.g. >1000 K). These limitations result in limited dynamic range.
Interferometric Rayleigh scattering technique for single and multiple-point velocity measurements is known in the art. However, due to low efficiency of the interferometer that is used to analyze the Rayleigh spectra, this technique is generally limited to time-averaged measurements of one component of velocity. Fabry-Perot and confocal interferometers have been used for Rayleigh signal analysis in conjunction with cooled CCDs (Charge-Coupled Devices), intensified CCDs (ICCDs), or photomultipliers arrays that image the interferogram. However, in many cases, thermal and vibration stabilization of the interferometer is required in order to improve the measurements precision in a harsh experimental environment. Furthermore, this requirement prevents those known systems from being configured as a compact system.
Another known technique can measure simultaneously three velocity components using combined instruments of different types (Rayleigh and LDV) and having different methods of obtaining the velocity. However, such a technique is complex and expensive to manufacture. Furthermore, the use of such complex components prevents this known system from being compact and configured on a single platform.
A search of patented known techniques reveals the following patents: U.S. Pat. No. 6,856,396 to McGuire discloses an airborne wind shear detection system that measures wind by determining the Doppler shift of backscattered radiation that is generated by an onboard laser system. The collected, scattered light is mixed with a sample from the transmitting laser using a dual differential Mach-Zehnder interferometer and a demodulator. U.S. Pat. No. 6,847,437 to Bruel et al. discloses a laser anemometer for determining the relative velocity between the anemometer and an ambient medium. U.S. Pat. No. 6,603,535 to McDowell discloses a stereo imaging velocimetry system and method that includes a camera for recording image processing and particle track determination. U.S. Pat. No. 6,542,226 to Wernet discloses a planar particle imaging and Doppler velocimetry system and method. The seeded flow field is illuminated with pulsed laser light source and the positions of the particles in the flow are recorded on CCD cameras. U.S. Pat. No. 6,522,397 to Barricau et al. discloses a method and a device for measuring the speed of at least one object by effect, whereby the light diffused by an object illuminated by a laser sheet is transmitted to a CCD video camera by special filtering means. U.S. Pat. No. 6,115,121 to Erskine discloses single and double superimposing interferometer systems. U.S. Pat. No. 5,708,495 to Pitz et al. discloses a method and an apparatus for determining the velocity of a gas flow wherein an image is recorded by a CCD (ICCD) camera. U.S. Pat. No. 5,351,116 to Barton et al. discloses a differential laser Doppler velocimeter that is based on the use of a modified fiber optic Sagnac interferometer. U.S. Pat. No. 5,333,044 to Shaffer discloses a florescent image tracking velocimeter (FITV) detects and measures the motion of small particles close to light scattering surfaces. The FITV includes an imaging camera. U.S. Pat. No. 5,088,815 to Gamier et al. discloses a laser device for measuring wind speeds at medium altitudes by using a Doppler effect. The device uses a Fabry-Perot interferometer.